1. Field of the Invention
The present invention relates to a display device. Specifically, the invention relates to a display device using an active matrix type display device such as liquid crystal display device.
2. Description of the Related Art
Rapid development has been made in recent years in a technique for manufacturing a semiconductor device, for example, a thin film transistor (TFT), which has a semiconductor thin film formed on an inexpensive glass substrate. This is because there is an increasing demand for active matrix type liquid crystal display devices (hereinafter referred to as liquid crystal display devices).
In the liquid crystal display device, several tens thousand to several million TFTs are arranged in matrix form in a pixel portion, and an electric charge going in and out of a pixel electrode connected to each TFT is controlled by a switching function of the TFT, so that an image is displayed.
Conventionally, thin film transistors using amorphous silicon formed on a glass substrate are arranged in the pixel portion.
A structure has come to be known in recent years in which quartz is utilized as a substrate and thin film transistors are fabricated from a polycrystalline silicon film. In this case, both of a peripheral driving circuit and the pixel portion are formed integrally on the quartz substrate.
Also known recently is a technique in which thin film transistors using a crystalline silicon film are formed on a glass substrate by laser annealing or other technologies.
FIG. 17 is a schematic structural view of a conventional active matrix type liquid crystal display device. In FIG. 17, reference numeral 20000 designates a source driver; 21000, a gate driver; and 22000, a pixel portion. The pixel portion 22000 is a circuit in which a plurality of TFTs 22100 are arranged in matrix form. Gate signal lines (G1, G2, . . . , G480) and source signal lines (S1, S2, . . . , S640) are respectively connected to gate electrodes and source electrodes of the pixel TFTs 22100. A pixel electrode is connected to a drain electrode of the TFT 22100. Reference numeral 22400 designates a storage capacitor. Here, the pixel portion includes (480×640) pixels. For convenience of explanation, symbols of (1, 1) to (480, 640) are given to the respective pixels.
In general, a substrate including a driving circuit and a pixel portion is called an active matrix substrate. A liquid crystal 22300 is held between the active matrix substrate and an opposite substrate (not shown) on one surface of which an opposite electrode is formed.
In the conventional active matrix type liquid crystal display device shown in FIG. 17, a clock signal (CK), a clock back signal (CLKB), a start pulse (SP), and a video signal (VIDEO) are inputted to the source driver, and a clock signal (CK), a clock back signal (CLKB), and a start pulse (SP) are inputted to the gate driver from the external.
Next, reference will be made to FIG. 18. FIG. 18 shows an operation timing chart of the conventional active matrix type liquid crystal display device shown in FIG. 17.
In the conventional active matrix type liquid crystal display device, the source driver 20000 sequentially generates timing signals in accordance with the clock signal (CLK), the clock back signal (CLKB), and the start pulse (SP), and outputs the timing signal to a sampling circuit in the source driver. The sampling circuit samples the externally inputted video signal (VIDEO) on the basis of the timing signal, and outputs to the corresponding source signal lines (S1, S2, . . . , S640).
Selection signals are sequentially supplied from the gate driver 21000 to the gate signal lines (G1, G2, . . . , G480). All TFTs connected to the gate signal line to which the selection signal is supplied are turned ON, and the source driver sequentially supplies the video signals to the source signal lines, so that an image signal is written in the TFT (that is, the liquid crystal and storage capacitor). Note that after the input of the selection signal to the gate signal line G1 is completed, the input of the selection signal of the gate signal line G2 is started. Then, after the input of the selection signal to the gate signal line G2 is completed, the input of the selection signal to the gate signal line G3 is started. In this way, the selection signals are sequentially inputted to the gate signal lines G1 to G480, and one frame period (TF) is completed.
For example, when the selection signal is inputted to the gate signal line G1, video signals (1, 1), (1, 2), . . . , (1, 640) are respectively inputted to the pixels (1, 1), (1, 2), . . . , (1, 640) connected to the source signal lines (S1, S2, . . . , S640). A period during which the video signals (1, 1), (1, 2), . . . , (1, 640) are inputted is called one line period (TL), and a period to a next one line period is called a horizontal retrace period (TH).
In such a conventional dot sequential active matrix type liquid crystal display device, since a load capacitor of the source signal line is large, it takes a time to write the video signal into the source signal line. Besides, since a time spent for writing the video signal into a storage capacitor of a pixel while the selection signal is inputted to the gate signal line varies for every pixel, especially in a pixel (for example, (1, 639), (1, 640), etc.) near the end of the selection signal, writing of the video signal into the storage capacitor of the pixel is made only in a part of the horizontal retrace period (TH). Thus, writing of the video signal is not sufficiently made into the storage capacitor of such a pixel, and as a result, degradation of display quality is caused.
As has just been described, there is fluctuation in a writing period of a video signal into a storage capacitor depending on in which pixel the signal is written, and hence some pixels are not allowed to have sufficient writing period.